The present disclosure generally relates to power amplifiers, and more specifically to predistortion of power amplifiers.
In general terms, an amplifier is a device for increasing the power of a signal. In an ideal world, an amplifier would take an input signal and linearly increase the power of the output signal. In the real world, the inherent output of typical power amplifiers, such as those used for wireless transmissions, is not linear with the input signal. While the output of the amplifier may linearly increase within limits, a point is reached when the amplifier becomes saturated and cannot produce any more output. This state is called clipping, resulting in distortion.
In the context of communication systems, unwanted nonlinearity may cause the transmitted signal spectrum spread into adjacent frequency channels, adversely interfering with adjacent ones. To mitigate this undesired result, a transmitter is often assigned a spectral mask, also known as a channel mask or transmission mask, which bounds the allowed transmission power at each frequency.
A concept related to communication efficiency is error vector magnitude (“EVM”), also referred to as receive constellation error or RCE. EVM is a measure used to quantify the performance of a transmitter or receiver. In an ideal world, a signal sent from a transmitter or received by a receiver would have all constellation points precisely at their ideal locations. In practice, system imperfections, such as phase noise or carrier leakage, cause the actual constellation points to deviate from the their ideal locations. The average deviation from the designated locations of the constellation points over all subcarriers and over large enough number of OFDM symbols is referred to as EVM. If high fidelity communication is desired, then having a low EVM is key. In general, as the data rate of transmission increases, additional bits per constellation point increases (i.e. the constellation increases), thus requiring better EVM. As the data rate of transmission decreases, the limits on desired EVM may increase but meeting the required spectral mask becomes more difficult.
Known techniques to reduce nonlinear effects of power amplifiers include feedback, feedforward, and predistortion. Such techniques are set forth in Joel L. Dawson, Power Amplifier Linearization Techniques: An Overview, Workshop on RF Circuits for 2.5G and 3G Wireless Systems, Feb. 4, 2001. The techniques aim at reducing the EVM, while complying with spectral emission mask constraints. In this manner, communication is enabled at high data rates and/or higher output powers, relative to uncompensated or less compensated amplifiers.
Predistortion involves modifying the signal before amplification in an effort to cancel out any distortion of the signal after amplification. Typical predistortion involves determining the characteristics of the nonlinearity of the output signal and inverting such characteristics in the input signal such that the combined transfer characteristic is linearized. Such linearization is usually performed digitally or discretely with subsequent conversion to an analog signal.
Basic concepts and mathematical foundations for digital predistortion are reported in Mohamed K. Nezami, Fundamentals of Power Amplifier Linearization Using Digital Pre-Distortion, 2004 High Frequency Electronics, Summit Technical Media, LLC. A polynomial based predistortion for linearization of an RF power amplifier is presented in D. Giesbers, S. Mann, K. Eccleston, Adaptive Digital Predistortion Linearization for RF Power Amplifiers, Electronics New Zealand Conference 2006. Predistortion and post-distortion correction of both a receiver and transmitter during calibration is set forth in U.S. Patent Application No. 2009/0316826.
Generally, predistortion is carried out as a compromise or trade-off between data rate of linearized output and a constraint of the spectral emission mask. In other words, known predistortion techniques fail to account properly for both EVM and spectral mask. To elaborate, in multi-rate systems such as IEEE 802.11, at high rates the spectral mask may be achieved but with poor EVM, while at low rates the spectral mask is violated while there is plenty of margin on EVM. At intermediate rates, the spectral mask may be slightly violated and EVM may be improved but with higher power. Not only are both EVM and spectral mask parameters not accounted for in today's predistortion techniques, but known predistortion techniques also focus on the predistortion optimization problem in the time domain perspective. Doing so has continually lead to unsatisfactory results.
A device and method of predistortion linearization that account properly for both EVM and spectral mask, and do so by focusing on the frequency domain rather than the time domain, are therefore desired.